Scientists from the University of Toronto have discovered what is believed to be the oldest known water on Earth — about two billion years — in northern Ontario, a finding that could point the way toward the possible existence of life on Mars.

The water, which contains a complex mix of elements that helped the team determine its age, was found some three kilometres underground in an active mine near Timmins. The discovery is significant because it suggests there could be a lot more water in Earth's deep crust than previously thought, says Oliver Warr, a post-doctoral fellow who helped collect and analyze the fluid samples from the mine. The team, led by geochemist Barbara Sherwood Lollar, previously identified billion-year-old water in the mine in 2013, at a depth of two and a half kilometres.

"Up to 30 per cent of the water in the crust could be at these kind of depths," Warr says. "At the moment, because we’ve only found one location, it’s extremely unique, but it could be a lot more widespread than anyone knows, and it’s just beneath our feet."

The finding also opens up new lines of inquiry in the search for life on other planets, particularly Mars, which has a rocky crust similar to Earth's and so may also harbour subterranean water.

Of course, Warr says there's much the water can tell us about our own planet as well, including what Earth was like two billion years ago and whether life exists at depths below two kilometres.

"We’re only really just scratching the surface," he says. "It turns out there’s this potentially massive store of water that we know nothing about, so the next step is to expand our search and try to figure out exactly what this water can tell us."

One thing the researchers already know about the water? You wouldn't want to drink it.

"It's eight times more salty than seawater; I can tell you right now, it would taste foul," laughs Warr.

Warr and Lollar presented their findings at the annual general meeting of the American Geophysical Union in San Francisco on Dec. 13; they'll be published in a future edition of the journal Nature.